1. Field of the Invention
The present invention relates to a method of determining the friction between the strand shell and the mold during continuous casting by means of an oscillating mold.
2. Description of the Related Art
The friction force between the mold wall and the strand shell is caused by a normal force which acts on the strand shell. In conventional molds with parallel planar mold plates, the normal force is produced primarily as a consequence of the ferrostatic pressure which is exerted by the molten steel in the strand interior against the strand shell. An additional portion of the friction force is caused by the conical position of the short sides of the mold.
In molds whose cross-section decreases towards the mold exit, for example, funnel-shaped molds, an additional normal force is produced by the reverse bending of the strand shell as it is transported through the mold.
In accordance with equation (1), the magnitude of the friction force FR depends on the normal force Fn as well as on the coefficient of friction xcexc.
FR=Fnxc3x97xcexcxe2x80x83xe2x80x83(1)
The coefficient of friction xcexc is determined primarily by the lubricating conditions between the strand and the shell. These conditions result from:
selection or quality of the lubricant, for example, casting powder or oil;
quantity of lubricant added;
condition of the lubricant with respect to viscosity, composition and texture;
heat removal from the mold plates;
relative speed between the strand shell at the mold.
The direction of the friction force changes periodically during the oscillating movement by 180xc2x0. In conventional molds, approximately the same friction values are achieved in both directions in the stationary state. On the other hand, because of the additional influence of the bending of the strand shell on the normal force, the friction forces in molds with decreasing cross-section are significantly higher during the phase of the positive strip than during the phase of the negative strip.
Modern continuous casting plants require the highest degree of availability of plant and process because unexpected changes in the casting process caused by lubrication can cause significant reductions of the quality of the cast product as well as interruptions of the operation. Consequently, it is an extremely important prerequisite for a problem-free operation to have a plant which is automated as completely as possible with permanent on-line monitoring of all essential operation data. It is possible in this manner to recognize the tendency of the development of defects early enough and to compensate them with appropriate counter measures.
For this purpose, it is especially necessary to continuously measure the frictional conditions between the strand shell and the mold in order to be able to obtain therefrom findings with respect to the state or condition of operation of the mold, so that the continuous casting process can be monitored and operationally optimized on the basis of this information.
The determination of the frictional conditions between the strand shell and the mold by using measurement technology is also of significant importance because the mold friction force acts on the strand as a pulsating disturbance variable.
Objectives of the on-line monitoring are:
improvement of the surface quality of the strand by optimizing the lubricating conditions;
continuous monitoring of the strand lubrication and the oscillation conditions combined with the possibility of a reaction to systematic changes;
timely warning of harmful events, such as, rupture, of which a recognizable tendency of changes of the mold friction is an indication.
Methods of determining the friction force between strand and mold are known in the prior art. They differ from each other primarily with respect to the selection of the measuring location and the measuring method used.
A method in which the mold is mounted on two load cells is described in a report entitled xe2x80x9cDetermination of Frictional Forces between Strand and Mouldxe2x80x9d, author M. Schmid, published in Concast News 12, 1973, pages 6 to 8. For preventing the occurrence of additional forces, for example, due to thermal deformation, it is necessary to guide the mold in low-friction needle bearings. In addition to the forces, the casting speed, the mold movement, the mold speed, the occurring forces as well as the mold acceleration are measured. The mold friction is computed from these data.
The document entitled xe2x80x9cOn the Importance of Mold Friction Control in Continuous Casting of Steelxe2x80x9d from xe2x80x9cFachberichte Hxc3xcttenpraxisxe2x80x9d Metallweiterverarbeitung, Vol. 20, No. 4, 1982, introduces a method of measuring the friction which is based on the measurements of the acceleration of the mold. For this purpose, a measuring head attached to the mold as well as a complicated electronic signal processing unit are required.
The document entitled xe2x80x9cEinsatz fortgeschrittener Verfahren zur Zustandsuberwachung von Kokillenhub-und-giexcex2maschinexe2x80x9d[Use of progressive methods for monitoring the conditions of mold lifting and casting machine], authors M. Perkuhn, E. Hxc3x6ffgen, H. J. Strodhoff, P. M. Frank, from xe2x80x9cVortrag zur Veranstaltung 3. Duisburger Stranggiexcex2tage am 7./8. Mxc3xa4rz 1991xe2x80x9d, a method is described for measuring the force at the eccentric lifting rod which uses as measuring values for the friction force measurement the force in the lifting rod of the eccentric as well as the motor current and the rate of rotation of the eccentric drive, the lifting travel path and the lifting frequency and the cooling water pressure. Using a model replacement system composed of masses, springs and damping units, the forces are computed at the same time which would have to be expected without the influence of the mold friction alone from the oscillating movement. The mold friction can then be determined from a comparison of the measured and expected forces in the lifting rod.
Some of the disadvantages of the methods described above are:
complicated structural and measuring requirements for obtaining clean measurement signals;
required retrofitting of mold and/or oscillating devices and of the mold guide means;
additional external application of measuring equipment which must be cared for, maintained and regularly monitored;
the required adjustment of the measurement values to vibration models or an interpretation by means of complicated electronic signal processing.
Therefore, starting from the prior art discussed above, it is the primary object of the present invention to provide a method of the above-mentioned type which avoids the disadvantages and difficulties discussed above and produces with relatively simple measurement technology and insignificant retrofitting of mold or oscillating device clean measurement signals, which does not require external measurement equipment and/or vibration models, and which produces continuously on-line the operation data required for monitoring a problem-free state of operation, particularly for the determination of the friction between the strand shell and the mold.
In accordance with the present invention, this object is met by measuring with the use of controlled double-acting hydraulic cylinders and with a predeterminable measuring frequency the pressures of always both chambers of all oscillating cylinders as well as the lifting positions of the pistons corresponding to these pressures, and by computing from these data the friction force acting at any point in time between the strand shell and the mold walls.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the descriptive matter in which there are described preferred embodiments of the invention.